Gnathotrichus retusus is primarily univoltine with a minor fraction of the population taking more than one year to complete the life cycle. From the spring to the summer, the development of brood from egg to adult took about 40 days or more in Douglas-fir logs. Brood production was significantly related to the gallery length. Boring activities were not consistently related with temperature, shifting from shallow sapwood to deep sapwood over time. Fungal staining was usually limited to the wood near the gallery entrance, and became darker in colour over time beginning in June. Abandonment of some galleries mainly occurred in June and July by females, and was probably induced by high temperatures. G. retusus preferred Douglas-fir stumps to western hemlock stumps, but both host tree species were equally suitable for G. retusus brood development. Significant differences were found among individual Douglas-fir and western hemlock stumps in attack density and brood production. Within a stump, attack density and brood production for Douglas-fir, and attack density for western hemlock increased from the stump top to the bottom. G. retusus emergence from Douglas-fir and western hemlock stumps and G. retusus and G. sulcat us flights began in late April when the daily maximum temperature reached 13.5°C, and peaked in late May. Brood emergence of G. retusus ceased, with rare exception, in late June. G. retusus had only one big peak flight while G. sulcatus had a small second peak flight in late July. Seasonal flights ceased in October when weekly mean maximum temperatures dropped below 15°C. Brood emergence, seasonal flights, and the sex ratios of brood ready to emerge and captured flying beetles were positively correlated with maximum temperature. G. retusus had a bimodal diurnal flight rhythm: a very small morning flight and a much larger dusk flight. Light intensity seemed to be a major stimulus in initiating the flight. However, diurnal flight might also be influenced by both temperature and relative humidity. Both G. retusus and G. sulcatus of both sexes responded significantly to ethanol or their own aggregation pheromones, (+)-sulcatol and (±)-sulcatol respectively, and ethanol was a synergist of (+)-sulcatol for G. retusus of both sexes. Alpha-pinene was neither a primary host attractant nor a synergist of aggregation pheromones. The sex ratios (female/male) of both species increased significantly from less than one for host chemicals alone to above one for treatments including aggregation pheromones. The sex ratio of G. sulcatus increased with increases in release rates of (±)-sulcatol. / Forestry, Faculty of / Graduate